Both Labour and the Republicans 'need to understand that rethinking without rearming is as ineffectual as rearming without rethinking'. Illustration: Satoshi Kambayashi

The bitter experience of defeat affects political parties in different ways. Most responses in most political cultures, however, are dominated by a combination of two competing instincts. The first, more prominent among parties whose principal goal is electoral success, is to rethink. The second, more marked in parties that see themselves primarily as crusades, is to rearm.

Rethinkers accept the voters who have rejected them may be on to something. They acknowledge that their own party may have become the problem. They are willing to look again at programmes, priorities and even principles in order to regain a sympathetic hearing from voters. Rearmers, on the other hand, tend to blame the voters not themselves for defeat, believe the only thing wrong is faintness of heart, and are often more focused on winning an argument in the party than with the voters. Both instincts have something to say. But getting the balance between them makes the difference between an effective response to defeat and a failed one.

Britain's Labour party remains instinctively, and rightly, more rethinker than rearmer. It is true that Labour has its rearming tendency, unapologetic for the past and energised afresh by hatred for the Tory tribe. All the same, Labour's dominant instinct after 2010 is still that its defeat was in part deserved and that it must start over again. That's partly a legacy of the controversial caution of 1990s New Labour, and partly a reflection of the pragmatic professionalism of most modern politicians. The Conservatives did exactly the same after 2005.

The US Republican party is almost the opposite. While not without rethinkers, the Republicans are above all a party of rearmers. Their response to defeat in 2008 was not that the voters might be telling them something from which they ought to learn. It was to get mad and get even with the Democrats. That's still the default mood, and it is why victories for the ultra-cautious Mitt Romney in the Iowa caucuses this week and, if the polls are right, in the New Hampshire primary next Tuesday, will not yet seal the deal in Romney's favour. Most Republicans want anyone but Romney.

These two parties operate in very different political worlds and embody very different traditions. British and US politics have never marched to the same drum. But each can learn from the other. Labour can learn from the Republicans that a party needs to love what it knows in order to sell its story. The Republicans can learn a much more enduring lesson from Labour that conviction without credibility is rarely a lasting answer. Both, in short, need to understand that rethinking without rearming is as ineffectual as rearming without rethinking.

If you are going to rethink you have to have thoughts. And you have to think them in public. That is why active rethinkers such as Maurice Glasman or Liam Byrne, both of whom fluttered the Labour dovecote this week in different ways, should be celebrated. Glasman may not be, as he is often dubbed, Ed Miliband's guru – I am assured he doesn't have a desk in the Labour leader's office and he has met Miliband for only three or four conversations in over a year. But he is a high-profile figure in an amorphous academic penumbra of Labour ideas people who are attempting to disentangle what they like and dislike in the Labour inheritance and to offer Miliband the rethought course that he properly wants.

Such work matters. It's terrible to dump on the rethinkers as soon as they open their mouths. But Glasman should be read not inhaled. His love of paradox and the strikingly made point can make him a liability, as his New Statesman chiding of Miliband this week – "no strategy, no narrative and little energy" – shows. His idiosyncratic taxonomy – Glasman has called David Cameron a socialist in charge of a liberal-led government – means he can confuse as often as he clarifies. Intellectuals in politics often do this: keep them at arm's length. But you only have to read Glasman's article in its entirety to see that he is full of ideas that are, at worst, a useful corrective to received mantras.

In the end, though, practical rethinkers have to get beyond the delights of irony and paradox in which Glasman too often wraps himself. They have to make better sense about what they can offer in government than they did before. That's what Byrne tried to do this week, invoking William Beveridge in support of a more incentivised and punitive benefits system, but also drawing on Glasman's emphasis on communitarian values. This isn't new, but it shouldn't be written off simply because it appears more suited to a Britain in which jobs are plentiful rather than one in which they are in short supply. It's right that rethinking hurts sometimes. Stay with it.

The Republican party's problem is that most of the time it doesn't rethink at all. Instead it just gets angrier. Live Free Or Die, the New Hampshire state motto that will be much quoted over the coming days, is a perfect rearmer slogan for a party whose instincts have now become dangerously apocalyptic. But without a dose of rethinking, the Republicans are heading nowhere. In politics as in nature, in Britain as in the US, the slogan that matters is Adapt or Die.